This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-192761, filed Jun. 27, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to an improvement in a transversal filter type elastic surface wave device.
2. Description of the Related Art
A transversal type elastic surface wave device is normally manufactured in a manner as described below. A metal film, such as an aluminum (Al) film, is formed on a piezoelectric substrate typically made of lithium tetraborate (LBO) and processed by lithography to produce comb-shaped electrodes and shield electrodes.
The comb-shaped electrodes include input side comb-shaped electrodes, output side comb-shaped electrodes and shield electrodes arranged between the input side comb-shaped electrodes and the output side comb-shaped electrodes.
In a transversal type elastic surface wave device, an elastic surface wave excited by the input side comb-shaped electrode is transmitted to the output side comb-shaped electrode. A bulk wave is generated at this time and propagated to the output side comb-shaped electrode. The generated bulk wave gets to the output side electrode after the surface wave that is propagated along the surface of the piezoelectric substrate gets there because it is propagated through the inside of the piezoelectric substrate.
As a result, it is found by analyzing the output signal of the output side comb-shaped electrode that a high frequency component is generated by the bulk wave at the higher frequency side of the pass band of the proper elastic surface wave. Therefore, the filter of the elastic surface wave device appears to be poorly effective for attenuation at the higher frequency side.
Measures taken for tackling this problem, include forming grooves on the back surface of the piezoelectric substrate opposite the side where the electrodes are arranged, processing in a rough finish on the back surface of the substrate or applying a sound absorbing material on the surface of the substrate. However, an elastic surface wave device comprising a substrate made of lithium tetraborate shows a large response to the bulk wave if compared with a device comprising a crystal substrate. Therefore, none of the above listed measures for suppressing the bulk wave are sufficient.
In view of the above identified circumstances, it is therefore an object of the present invention to provide an elastic surface wave device that can suppress the high frequency component generated by unnecessary waves, including the bulk wave, to a practically feasible level and shows an excellent band pass characteristic.
Another object of the present invention is to provide an elastic surface wave device that can suppress the high frequency component generated by unnecessary waves including the bulk wave to a practically feasible level and be prepared by using a substrate made of lithium tetraborate.
According to the invention, the above objects and other objects are achieved by providing an elastic surface wave device comprising:
a main elastic surface wave filter having input side comb-shaped electrodes, shield electrodes and output side comb-shaped electrodes formed on a piezoelectric substrate; and
an auxiliary elastic surface wave filter connected in parallel with the main elastic surface wave filter and having input side comb-shaped electrodes, shield electrodes and output side comb-shaped electrodes formed on the piezoelectric substrate;
the auxiliary elastic surface wave filter having a canceling high pass band for reducing the high frequency component generated by a bulk wave at the higher frequency side of the pass band of the main elastic surface wave filter on the frequency axis.